This application is based upon, and claims the benefit of priority of, prior Japanese Patent Application 2001-169673, filed on Jun. 5, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a multi-layer current-carrying member (a brush portion or a commutator portion) for a direct-current motor in a fuel pump, a method for producing the member, and a fuel pump employing the member.
2. Description of Related Art
Engines in automobiles, and the like, are normally equipped with low pressure fuel pumps for pumping fuel from a fuel tank to a delivery pipe or to a high pressure pump of a fuel injection valve or device. Such a fuel pump is generally driven by a direct-current (DC) motor using a battery or other power supply source. In the case of the DC motor, it is necessary to keep a direction of a current constant with a rotational position of an armature coil, and for this reason, rectification is carried out by a brush and a commutator.
Additionally, resistance to fuel erosion (resistance to a fuel that causes erosion) is required for a fuel pump, whether it is an in-tank type or other type, and, in particular, for its DC motor. Japanese Laid-Open Patent Publication No. Hei. 8-308183 discloses a commutator (carbon commutator segments) containing a carbon layer in a sliding portion for a brush. Further, the publication describes that by forming a metal carbon layer beneath the carbon layer, the bonding to a conductive metal terminal (riser) can be ensured. The conductive metal terminal is connected to the metal carbon layer at one end thereof, and to the armature coil at the other end. Similar descriptions are disclosed in Japanese Laid-Open Patent Publication No. Hei. 9-46978 and International Publication No. WO99/08367.
The metal carbon layers disclosed in Japanese Laid-Open Patent Publication Nos. Hei. 8-308183 and Hei. 9-46978 are formed by sintering a copper powder with a carbon powder. Also, the metal carbon layer disclosed in the International Publication No. WO99/08367 is formed by sintering a mixed powder of a copper powder, a tin powder, and a carbon powder. International Publication No. WO99/08367 describes bonding between the carbon layer and the conductive metal terminal as being improved due to a liquid phase of the tin powder having a low melting point. A composition ratio of copper to tin in this publication is about Cu-10 Sn (wt %).
The inventor of the present invention, however, has found that a current-carrying member, such as a commutator and a brush, using copper or copper alloy with a high content of copper has poor resistance to a fuel. To be more specific, the current-carrying member has poor resistance to erosion because of sulfur in the fuel. Erosion of the current-carrying member is not acceptable because it deteriorates the durability and performance of the fuel pump. Hence, the current-carrying member is required to have satisfactory resistance to sulfidation.
The present invention was devised in view of the foregoing disadvantages. Therefore, it is an object of the present invention to provide a sulfidation-resistant current-carrying member for a DC motor in a fuel pump. It is another object of the present invention to provide a method for producing such a fuel pump. The present invention has yet another object which is to provide a fuel pump employing the current-carrying member.
(Current-carrying Member for a DC Motor in a Fuel Pump)
In order to achieve the above and other objects, sulfidation-resistant brass is used as the metal in the metal carbon layer of a current-carrying member composed of a carbon layer and a metal carbon layer. That is, a current-carrying member for a DC motor in a fuel pump of the present invention comprises a brush having a sliding surface at one end and fed with a current from a power supply source. Additionally, the member has a commutator with a sliding surface that slides on a sliding surface of the brush for feeding an armature coil with the current received from the brush. At least one of the brush and the commutator is composed of a carbon layer having carbon as a principal component and forming the sliding surface at one end, and a metal carbon layer fixedly bound with the carbon layer at the other end and comprising 55 to 90 wt % of metal having brass as a principal component and a remaining portion having carbon as a principal component.
According to the current-carrying member of the present invention, because metal in the metal carbon layer has brass as a principal component, resistance to sulfidation is enhanced, making it possible to enhance durability of the current-carrying member, and hence, that of the fuel pump. The reason why a weight ratio of the metal in the metal carbon layer (out of a total of 100 wt %) is set to a range from 55 to 90 wt % is as follows. A weight ratio less than 55 wt % is not preferable because a specific resistance value of the metal carbon layer increases.
Generally, a conductive metal terminal is connected to the metal carbon layer, and the armature coil is fed with a current through the conductive metal terminal. For this reason, it is preferable that the metal carbon layer has a small resistance value. Also, a weight ratio of the metal less than 55 wt % is not preferable, because the metal carbon layer has poor strength, which deteriorates the retentivity of the current-carrying member. In particular, in a case where the conductive metal terminal is connected to the metal carbon layer mechanically (for example, in the case of press-fitting or fastening), the metal carbon layer needs to have the strength of a certain or greater degree in order to secure a stable connection therebetween.
Likewise, a weight ratio of the metal higher than 90 wt % is also not preferable because the bonding strength in the vicinity of the boundary of the carbon layer and the metal carbon layer becomes poor. When the bonding strength becomes poor, an overall resistance value increases, which deteriorates the retentivity of the current-carrying member. Further, when a weight ratio of the metal exceeds 90 wt %, the metal carbon layer becomes too rigid and hard. Hence, for example, in a case where the conductive metal terminal is connected to the metal carbon layer by press-fit or the like, the machining load required to achieve the press-fit, or the like, increases. Then, the load reaches the carbon layer, having a relatively poor toughness, which may possibly cause cracking or the like in the carbon layer.
In view of the foregoing, it is preferable to set a weight ratio of the metal to a range from 55 to 90 wt % and more preferably to a range from 70 to 80 wt %. The current-carrying member of the present invention includes a type in which the conductive metal terminal is embedded into the metal carbon layer and the metal carbon layer and the conductive metal terminal are integrally sintered.
The metal carbon layer is composed of, for example, a porous metal skeleton having brass as a principal component, and a filling material present in the porous metal skeleton having carbon as a principal component. The metal carbon layer may be integrally formed by sintering a mixture of a brass powder, a carbon powder, etc. or formed by filling foamed metal (porous metal skeleton) prepared in advance to have brass as a principal component with a filling material having carbon as a principal component.
Brass used in the metal carbon layer is an alloy having Cu and Zn as principal components. For example, a copper alloy containing 25 to 45 wt % of Zn on a basis of 100 wt % of brass is favorable as brass. Zn has a larger sulfide-producing free energy than Cu, and is an effective sulfidation-resistant element. When a weight ratio of Zn is less than 25 wt %, resistance to sulfidation of the metal carbon layer becomes insufficient. A weight ratio of Zn higher than 45 wt % is not preferable, because of the xcex2 phase precipitates, which deteriorate the workability of the metal carbon layer.
When Zn is added up to 45 wt %, the strength is enhanced with the addition of Zn, and when Zn is added up to 40 wt %, the elongation is enhanced in comparison with a case using a pure metal powder, which provides advantages in connecting the conductive metal terminal to the metal carbon layer by press-fitting or the like. Hence, a weight ratio of Zn is preferably in a range from 25 to 45 wt % and more preferably in a range from 30 to 40 wt %. Therefore, the carbon layer is sulfidation-resistant.
In the present specification, the term xe2x80x9ccurrent-carrying memberxe2x80x9d includes, but is not limited to, commutator segments and a unit of a brush, and can be broadly interpreted as the commutator segments and the brushes plus peripheral members, such as the conductive metal terminals (risers, pig tails, etc.), connected thereto. In short, the current-carrying member means a unit member or a device necessary to feed the armature with a current.
The aforementioned xe2x80x9cmetalxe2x80x9d has brass as a principal component, but it may be brass itself, or include an alloy element, such as Sn, in addition to brass. xe2x80x9cCarbonxe2x80x9d is a carbonaceous material having a sliding property and conductivity, and is primarily graphite. It should be appreciated, however, that the carbon layer is not limited to a layer made of 100% of carbon, and may include a binder or other kinds of treatment agent. It is needless to say that xe2x80x9cmetalxe2x80x9d, xe2x80x9ccarbonxe2x80x9d, xe2x80x9ccarbon layerxe2x80x9d, and xe2x80x9cmetal carbon layerxe2x80x9d may include unavoidable impurities.
(Method for Producing a Current-carrying Member)
(1) A Producing Method of the Current-carrying Member
The above-described current-carrying member is produced by a producing method of the present invention as follows. That is, a method for producing a current-carrying member for a DC motor in a fuel pump of the present invention is a method for producing a current-carrying member including a brush having a sliding surface at one end and fed with a current from a power supply source. Additionally, a commutator is provided with a sliding surface that slides on the brush sliding surface for feeding an armature coil with the current received from the brush, with at least one of the brush and the commutator being composed of a carbon layer forming the sliding surface at one end, and a metal carbon layer fixedly bound with the carbon layer at the other end. The method entails a filling step of filling a green compact mold first with one of: (a) a carbon powder having carbon as a principal component to form the carbon layer, or (b) a mixed powder comprising 55 to 90 wt % of a metal powder having brass as a principal component mixed with a carbon powder having carbon as a principal component to form the metal carbon layer. Next is a molding step of forming a molded green compact by compacting the powders after the filling step and a sintering step of sintering the carbon layer and the metal carbon layer while they are fixedly bound with each other by heating the molded green compact.
According to the producing method of the present invention, it is possible to produce the above-described current-carrying member efficiently. The reason why the weight ratio of the metal powder is set to a range from 55 to 90 wt % on a basis of 100 wt % of the metal carbon layer is the same as given above. The carbon powder may contain a binder made of a thermo-setting resin, such as phenol resin, if required. The same can be said as to the mixed powder. The metal powder may be a brass powder or a metal mixed powder of a zinc powder mixed with a copper powder.
In the filling step, it is preferable to carry out a compacting step of compacting a carbon powder forming the carbon layer or the mixed powder forming the metal carbon layer. Both are filled during the preceding filling step. According to this step, it becomes easier to regulate the thickness of the carbon layer or the metal carbon layer. Incidentally, as to the metal powder, a water atomized powder having a strong oxide film on the surface thereof is favorable as the brass powder. The reason why is as follows. That is, although Zn readily evaporates from the metal powder during the sintering step, when the brass powder having a strong oxide film on the surface is used, a loss of Zn can be controlled and prevented by the oxide film.
Generally, according to the gas atomization method, molten metal dropped from a nozzle is blown off by a reducing gas to be divided into particles. By using water in this blowing method, a strong oxide film is formed on the surface of the brass when it is cooled from a high temperature. A copper alloy powder containing 25 to 45 wt % of Zn on a basis of 100 wt % of brass is favorable as the brass powder. The reason why is the same as given above.
Also, it is favorable that the metal powder contain a tin powder. Tin has a lower melting point than brass or the like, and the tin powder begins to melt first in the sintering step. As a result, the tin powder and the brass powder or the like undergo partial molten bonding (liquid phase sintering) at a relatively low temperature. Also, Cu melts into molten Sn, which facilitates easier dispersion of one in the other. Accordingly, a uniform metal composition is attained in the metal carbon layer, thereby making it possible to obtain a metal carbon layer in which the metal powder is firmly bound. For the same reason, the metal powder preferably contains a phosphorus compound powder instead of or in addition to the tin powder.
It is preferable to adjust the size of a green compact mold and a quantity of the filled carbon powder or metal powder by taking into account the thermal contraction or the like during sintering. Also, it is sufficient that the metal carbon layer has a uniform composition across the layer, but a functional gradient may be conferred by changing compositions of the mixed powder step by step, if required.
(2) Additional Producing Method of the Current-carrying Member
The method for producing the current-carrying member may be as follows. That is, a method for producing a current-carrying member for a DC motor in a fuel pump of the present invention is a method for producing a current-carrying member comprising a brush having a sliding surface at one end and fed with a current from a power supply source. Additionally, a commutator having a sliding surface that slides on the sliding surface of the brush for feeding an armature coil with the current received from the brush is present while at least one of the brush and the commutator are composed of a carbon layer and form the sliding surface at one end. A metal carbon layer is fixedly bound with the carbon layer at the other end. The method comprises: a metal carbon molding step of forming a molded metal carbon green compact by filling cavities in 55 to 90 wt % of foamed metal having brass as a principal component to form the metal carbon layer with a filling material having carbon as a principal component; a carbon molding step of forming a molded carbon green compact by filling a green compact mold with a carbon powder having carbon as a principal component to form the carbon layer, followed by compacting; and a sintering step of sintering the metal carbon layer with the carbon layer fixedly with the layers bound to each other by depositing the molded metal carbon green compact and the molded carbon green compact.
A weight ratio of 55 to 90 wt % given to the foamed metal is based on 100 wt % of the molded metal carbon green compact. Because the porous metal skeleton is already formed, the metal mixed powder need not be adjusted, which makes it easier to control the temperature during the sintering step. The foamed metal is filled with the filling material by, for example, dipping the foamed metal in a liquid treatment prepared by dissolving carbon into a solvent, or by applying a spray of the liquid treatment onto the foamed metal followed by drying.
A plurality of thin sheets of foamed metal filled with carbon may be prepared so that the metal carbon layer of a desired thickness is obtained by depositing and pressing these thin sheets. It is efficient to place the molded metal carbon green compact in the green compact mold to form the molded carbon green compact integrally with the molded metal carbon green compact in the carbon molding step, so that the step of depositing the molded metal carbon green compact and the molded carbon green compact can be omitted.
The descriptions as to a weight ratio of the foamed metal, the carbon powder, brass, etc. also apply in this case.
(Fuel pump)
The above-described current-carrying member can be applied to a DC motor regardless of the type of fuel pump that the motor drives. By employing this current-carrying member, it is possible to enhance the durability of the fuel pump. Hence, the present invention can be interpreted as a fuel pump employing the above-described current-carrying member.
That is, a fuel pump of the present invention is a fuel pump employing a current-carrying member for a DC motor, the current-carrying member including: a brush having a sliding surface at one end and fed with a current from a power supply source; and a commutator having a sliding surface that slides on the sliding surface of the brush for feeding an armature coil with the current received from the brush. Furthermore, at least one of the brush and the commutator is composed of a carbon layer having carbon as a principal component and forming the sliding surface at one end, and a metal carbon layer fixedly bound with the carbon layer at the other end and comprising 55 to 90 wt % of metal having brass as a principal component and a remaining portion having carbon as a principal component.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.